[U-03] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U03-P03] Radiocarbon dating of fossil pollen to estimate the carbon sequestration rate of peat

*Ren Tsuneoka^1,2, Wataru Sakashita³, Ota Kosuke⁴, Yosuke Miyairi¹, Reisuke Kondo¹, Yusuke Yokoyama^1,2,5,6,7 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 3.Forestry and Forest Products Research InstitutF, 4.Research Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology, 5.Graduate Program on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 6.Biogeochemistry Program, Japan Agency for Marine-Earth Science and Technology, 7.Research School of Physics, The Australian National University)

Keywords:Radiocarbon dating, Pollen fossils, Flow cytometry, Peat, Carbon sequestration

Peatlands play a critical role in mitigating climate change, as they are one of the largest carbon sinks among terrestrial ecosystems. A number of attempts have been made to determine the long-term carbon sequestration rate of peat using radiocarbon (¹⁴C) dating. However, the bulk peats generally contain various organic materials, which can make bulk ¹⁴C dating unreliable. ¹⁴C dating on macrofossils (e.g., leaves, twigs) provide accurate ages, but cannot always be found from peat sequences, and therefore cannot determine all ages at targeted depths. In this study, we focused on pollen fossils, which are generally ubiquitous in terrestrial sediments, and suitable for ¹⁴C dating. It is necessary to collect hundreds of thousands of pollen grains for ¹⁴C analysis, and conventional pollen extraction methods from sediment (i.e., physicochemical treatments, hand-picking) have certain drawbacks, such as the possibility of incomplete removal of impurities and excessively long processing times. In recent years, however, several studies have attempted to apply flow cytometry techniques to extract pollen fossils from lake sediments for ¹⁴C analysis. This technique allows for more efficient pollen screening compared to conventional methods. In this study, we hypothesized that flow cytometry can also be applied to peat soils. The objective of this study is 1) to verify the validity of the pollen fossil ¹⁴C ages obtained using flow cytometry and 2) to estimate the carbon sequestration rate based on those ages.
¹⁴C analyses of the pollen fossils were performed at four depths throughout a peat core obtained from a wetland in eastern Hokkaido. Comparisons between the tephra and pollen fossils ¹⁴C ages revealed that the ¹⁴C ages of pollen fossils are consistent with the age of tephra. Additionally, our results showed that the pollen fossils ¹⁴C ages were about 500-1200 years older than the bulk ¹⁴C ages of the adjacent layers. This offset between bulk and pollen ¹⁴C ages may be due to the contribution of younger ¹⁴C from overlying layers to the lower sediments (e.g., fine roots turnover). In this presentation, we will also report on the carbon sequestration rate calculated from the preliminary results of pollen fossils ¹⁴C ages.